Effects of Aluminum Content and Heat Treatment on Gamma Prime Structure and Yield Strength of Inconel Nickel-Chromium Alloy 706

Abstract

An intermediate heat treatment at 1550°F used to improve the rupture ductility of INCONELY: alloy 706 had a varied effect on room-temperature yield strength of the alloy. The effect seemed to depend on very small differences in aluminum content. A laboratory investigation was conducted to determine the effects of variations in aluminum content and heat treatment on the yield strength of alloy 706. Samples having aluminum contents from 0.033% to 0.90% were studied after being subjected to four different heat treatments. The investigation produced an explanation of the variations in yield strength in terms of changes in structure of gamma prime, gamma prime/ lattice mismatch, and changes in modulus of elasticity. It was found that material age-hardened without the intermediate treatment is in a slightly underaged condition, and that yield strength of such material is a function of gamma prime particle properties. It was also found that material given the 1550°F intermediate treatment and then age-hardened is slightly overaged. The yield strength of overaged material is not a function of gamma prime properties but depends instead on matrix shear modulus and interparticle spacing of the gamma prime. INTRODUCTION INCONEL nickel-chromium-iron alloy 706 is a relatively new precipitation-hardenable superalloy. It has the high strength and good weldability of the widely used INCONEL alloy 718 but with greatly improved machinability. During development of alloy 706, it was found that rupture ductility could be improved by a 1550°F treatment prior to the final age-hardening treatment. This treatment, however, had a varied effect on room-temperature yield strength. The effect seemed to depend on very small differences in aluminum content. Ma,terial directly aged without the 1550°F treatment had high yield strength at low aluminum contents, and yield strength decreased as aluminum content was increased up to about 0.65%. With the 1550°F preage treatment, however, material with low aluminum content had low yield strength, and yield strength increased with increasing aluminum up to about 0.4%. To explain these changes in yield strength, a laboratory investigation was performed to study the effects of aluminum content and heat trea.tment on the properties and structure of alloy 706. *INCONEL is a Registered Trademark of The International Nickel Company, Inc.